A monochromatic red (R), green (G), and blue (B) light source having well color-balanced white light output is a very desirable light source for many display applications, because it is known to be more efficient compared to other conventional incoherent xe2x80x9cwhite lightxe2x80x9d light sources with red, green and blue filters. For example, used in conjunction with 3 channel, transmissive Liquid Crystal Display (LCD) panel modulator based projection display systems, the monochromatic R,G,B light source producing well color-balanced white light can achieve a significantly higher optical efficiency than conventional high-intensity incoherent light sources, such as Xenon lamps or Metal Halide lamps with red, green and blue filters.
Monochromatic R,G,B light can originate from lasers producing a visible output at red, green and blue wavelengths. However, previous attempts to develop such lasers had the following shortcomings:
Gas lasers have an unacceptably low wall plug efficiency and require frequent maintenance.
Some of the 3 primary wavelengths generated by the R,G,B lasers may not be suited for display use.
Some spatial light modulators used in display applications work well with continuous wave (cw) R,G,B lasers. The others need pulsed R,G,B lasers. It has been known to be very difficult to generate suitable R,G,B colors efficiently from cw laser designs, and coupling of spatial light modulators, which require cw light sources, with pulsed lasers has in the past proven to be difficult due to an unacceptably long pulse duration.
Another serious problem associated with past R,G,B lasers is laser speckle, which can be caused by a random interference pattern resulting from the reflection or transmission of highly coherent light from (or through) an optically rough surface (one whose local irregularities in depth are greater than one quarter of a wavelength from a laser). The speckle is very annoying to look at, and past attempts employed to minimize the speckle have been to disturb the phase coherency associated with each wave front, for example, by vibrating the projection screens.
The invention relates to a monochromatic laser system; display systems employing the light source; and the method of providing the light source and displays. The invention is directed to a monochromatic R,G,B laser adapted for display applications, and particularly, LCD display systems. The light source includes a single or multiple laser master oscillator with a laser medium, for example, a neodymium (Nd)-containing laser medium, particularly, Nd:YVO4, neodymium yttrium vanadate, to provide a signal wave laser radiation, e.g., of about 1342 nm, 1047 nm and 1064 nm of a selected pulse width and pulse rate, and single or multiple laser power amplifier to amplify the power of the laser radiation. The light source includes single or multiple, preferably all solid state, non-linear frequency converters to receive and convert the amplified signal wave laser radiation to produce a green wavelength, e.g., about 532 nm or 523.5 nm; with slight spatial incoherency to reduce speckle; red wave length, e.g., about 626 nm to 629 nm with narrow spectrum incoherency to reduce speckle; and blue wave length, e.g., about 447 nm to 452 nm, which R,G,B wavelengths are particularly useful for color display purposes, such as when the light source is coupled with, and used in a three LCD display system, as hereinafter described.
The invention is also directed to a method by which speckle noise resulting from laser sources may be reduced. Laser light produces a speckle noise, since it has a long coherent length. Speckle noise of lasers can be reduced by introducing spatial incoherency in the green spectral range, which is most visible to the human eye, and by multimode operation and producing spectrum incoherency in the red spectral range. Speckle noise in the blue portion of the spectrum is typically not a significant issue due to the reduced sensitivity of the eye.
Pulsed operation of the R,G,B laser is advantageous, since pulsed operation permits the use of various non-linear frequency conversion processes, such as Optical Parametric Oscillation (OPO) and Sum Frequency Mixing (SFM) to produce visible wavelengths from the fundamental laser radiation.
The invention is also directed to display systems employing the monochromatic R,G,B laser, particularly for LCD display systems, since LCD panel (one of spatial light modulators) does not require pulse width modulation, the R,G,B laser light source may be coupled to three LCD panels (one panel for each primary color) to create a display system.
The invention presents a monochromatic R,G,B light source which incorporates:
a solid state laser design which produces very high electrical to optical efficiency;
a laser cavity design that operates at high pulse repetition rate (40-80 kHz range) and produces very short pulse (FWHM xcx9c7 ns range), or another design that operates at very high pulse repetition rate ( greater than  greater than 1 MHz range) and produces ultra short pulse width (FWHM xcx9c100 ps range) with active modelock mechanism, which allows this monochromatic R,G,B light source to be coupled with many different spatial light modulators, which requires R,G,B light source that operates in cw or quasi-cw mode; and
a green and red laser design which produces multi-mode laser radiation (for green, M2xcx9c10). The multi-mode operation of the laser helps increase the spatial incoherency, thus, reducing the speckle effect.
The invention will be described for the purposes of illustration only in connection with certain illustrated embodiments; however, it is recognized that those persons skilled in the art may make various additions, improvements, modifications and changes to the illustrated embodiments, without departing from the spirit and scope thereof.